diff --git a/doc/sphinx/arm/hooks-ha.rst b/doc/sphinx/arm/hooks-ha.rst index e7746289d3..3f099a7362 100644 --- a/doc/sphinx/arm/hooks-ha.rst +++ b/doc/sphinx/arm/hooks-ha.rst @@ -69,8 +69,8 @@ server synchronizes the database first. The secondary server waits for the primary server to complete the lease database synchronization before it starts the synchronization. -In the hot-standby configuration, one of the servers is also designated -as "primary" and the second as "standby." However, during normal +In the hot-standby configuration, one of the servers is designated +as "primary" and the other as "standby." However, during normal operation, the primary server is the only one that responds to DHCP requests. The standby server receives lease updates from the primary over the control channel; however, it does not respond to any DHCP @@ -606,11 +606,11 @@ set the ``max-unacked-clients`` to 0. The ``delayed-updates-limit`` parameter was introduced in Kea 1.9.4. It is used to enable or disable the use of the communication recovery procedure and controls server's behavior in the ``communication-recovery`` -state which were introduced in the same release. This parameter may +state which was introduced in the same release. This parameter may only be used in the load balancing mode. If the server is in the ``load-balancing`` state and it experiences -communication issues with a partner (heartbeat or lease update fail), +communication issues with its partner (heartbeat or lease update fail), the server transitions to the ``communication-recovery`` state. In this state the server keeps responding to DHCP queries but it does not send lease updates to the partner. The lease updates are queued until the @@ -637,27 +637,27 @@ of the same IP addresses or delegated prefixes to different clients by respective servers. By entering the intermediate ``communication-recovery`` state these problems are avoided. -If the server in the ``communication-recovery`` state re-establishes -communication with the partner, it will try to send all outstanding -lease updates to it. This is done synchronously and may take considerable -amount of time before the server transitions to the ``load-balancing`` -state and resumes normal operation. The maximum number of lease updates -which can be queued in the ``communication-recovery`` state is controlled -by the ``delayed-updates-limit``. If the limit is exceeded, the server -stops queuing lease updates and will perform full database synchronization -after re-establishing the connection with the partner instead of -sending outstanding lease updates before transitioning to -``load-balancing`` state. Even if the limit is exceeded, the server -in the ``communication-recovery`` state remains responsive to the DHCP -clients. +If a server in the ``communication-recovery`` state re-establishes +communication with its partner, it will try to send the partner all +of the outstanding lease updates the server has queued. This is done +synchronously and may take considerable amount of time before the server +transitions to the ``load-balancing`` state and resumes normal operation. +The maximum number of lease updates which can be queued in the +``communication-recovery`` state is controlled by the ``delayed-updates-limit``. +If the limit is exceeded, the server stops queuing lease updates and +will perform full database synchronization after re-establishing the +connection with the partner instead of sending outstanding lease updates +before transitioning to ``load-balancing`` state. Even if the limit is +exceeded, the server in the ``communication-recovery`` state remains +responsive to the DHCP clients. -It is preferred to set higher values of ``delayed-updates-limit`` when +It may be preferable to set higher values of ``delayed-updates-limit`` when there is a risk of prolonged communication interruption between the servers and the lease database is large. This would avoid costly lease database synchronization. On the other hand, if the lease database is small the time required to send outstanding lease updates may be longer than lease database synchronization. In such cases it -may be better to use lower value, e.g. 10. The default value is 100 +may be better to use a lower value, e.g. 10. The default value is 100 which seems to be a reasonable compromise and should work well in most deployments with moderate traffic.